In recent years, from the viewpoint of global environmental protection, an increasing number of boiler type heaters that burn fossil fuels for heating have been replaced, even in cold climate areas, by heat pump type air-conditioning apparatuses that use air as a heat source.
The heat pump type air-conditioning apparatus can provide efficient heating, because heat is supplied from air as well as from electricity input to a compressor.
However, when the outdoor air temperature drops, frost forms on an outdoor heat exchanger serving as evaporator, and hence a defrosting operation needs to be performed to melt the frost on the outdoor heat exchanger.
The defrosting operation may be done by reversing the refrigeration cycle. However, this leads to discomfort, because indoor heating is suspended during the defrosting operation.
To perform a heating operation during defrosting, a technique that involves dividing the outdoor heat exchanger has been proposed. In this technique, during defrosting of one of the resulting heat exchangers, the other at least one heat exchanger serves as an evaporator to receive heat from air for heating (see, e.g., Patent Literatures 1, 2, and 3).
In the technique described in Patent Literature 1, an outdoor heat exchanger is divided into two heat exchanger units. For defrosting of one of the heat exchanger units, an electronic expansion valve provided upstream of the heat exchanger unit to be defrosted is closed. Then, a solenoid on/off valve in a bypass pipe that allows refrigerant to flow from a discharge pipe of a compressor to the inlet of the heat exchanger unit is opened, so that a part of high-temperature refrigerant discharged from the compressor directly flows into the heat exchanger unit to be defrosted. When the defrosting of one of the heat exchanger units ends, defrosting of the other heat exchanger unit is performed.
In this case, the defrosting of the heat exchanger unit to be defrosted is performed, with the pressure of refrigerant in the heat exchanger unit being equal to the suction pressure of the compressor (low-pressure defrosting).
The technique described in Patent Literature 2 involves using a plurality of heat source units and at least one indoor unit. Only in the heat source unit that includes a heat-source-side heat exchanger to be defrosted, the connection of a four-way valve is made opposite to that during heating, so that the refrigerant discharged from the compressor directly flows into the heat-source-side heat exchanger.
In this case, defrosting of the heat-source-side heat exchanger to be defrosted is performed, with the pressure of refrigerant in the heat-source-side heat exchanger being equal to the discharge pressure of the compressor (high-pressure defrosting).
In the technique described in Patent Literature 3, an outdoor heat exchanger is divided into a plurality of parallel heat exchangers. Then, a part of high-temperature refrigerant discharged from a compressor is reduced in pressure and allowed to alternately flow into the parallel heat exchangers. Thus, by alternately defrosting the parallel heat exchangers, heating can be continuously performed without reversing the refrigeration cycle. The refrigerant supplied to the parallel heat exchanger to be defrosted is injected from an injection port of the compressor.
In this case, the defrosting of the parallel heat exchanger to be defrosted is performed, with the pressure of refrigerant in the parallel heat exchanger being lower than the discharge pressure of the compressor and higher than the suction pressure of the compressor (i.e., the pressure equivalent to a saturation temperature of slightly higher than 0 degrees Celsius) (medium-pressure defrosting).